Genetic analyses in Drosophila have been extremely useful in dissecting the complexity of developmental pathways and identifying interacting loci. However, understanding the precise nature of the processes that underlie genetic interactions requires a knowledge of the protein products of the genes in question.
The vertebrate central nervous system is an intimate mixture of different cell types, almost all generated from the same source--the neurogenic epithelium that forms the neural plate and subsequently the neural tube. What are the mechanisms that control neurogenesis in this sheet of cells, directing some to become neurons while others remain non-neuronal? The answer is virtually unknown for vertebrates, but many of the cellular interactions and genes controlling cell fate decisions during neurogenesis have been well characterized in Drosophila (Campos-Ortega, 1993, J. Neurobiol. 24:1305-1327). Although the gross anatomical context of neurogenesis appears very different in insects and vertebrates, the possibility remains that, at a cellular level, similar events are occurring via conserved molecular mechanisms. Embryological, genetic and molecular evidence indicates that the early steps of ectodermal differentiation in Drosophila depend on cell interactions (Doe and Goodman, 1985, Dev. Biol. 111:206-219; Technau and Campos-ortega, 1986, Dev. Biol. 195:445-454; Vassin et al., 1985, J. Neurogenet. 2:291-308; de la Concha et al., 1988, Genetics 118:499-508; Xu et al., 1990, Genes Dev. 4:464-475; Artavanis-Tsakonas, 1988, Trends Genet. 4:95-100). Mutational analyses reveal a small group of zygotically-acting genes, the so called neurogenic loci, which affect the choice of ectodermal cells between epidermal and neural pathways (Poulson, 1937, Proc. Natl. Acad. Sci. 23:133-137; Lehmann et al., 1983, Wilhelm Roux's Arch. Dev. Biol. 192:62-74; Jurgens et al., 1984, Wilhelm Roux's Arch. Dev. Biol. 193:283-295; Wieschaus et al., 1984, Wilhelm Roux's Arch. Dev. Biol. 193:296-307; Nusslein-Volhard et al., 1984, Wilhelm Roux's Arch. Dev. Biol. 193:267-282). Null mutations in any one of the zygotic neurogenic loci--Notch (N), Delta (D1), mastermind (mam), Enhancer of Split (E(spl), neuralized (neu), and big brain (bib)--result in hypertrophy of the nervous system at the expense of ventral and lateral epidermal structures. This effect is due to the misrouting of epidermal precursor cells into a neuronal pathway, and implies that neurogenic gene function is necessary to divert cells within the neurogenic region from a neuronal fate to an epithelial fate.
Neural precursors arise in the Drosophila embryo from a neurogenic epithelium during successive waves of neurogenesis (Campos-Ortega & Hartenstein, 1985, The embryonic development of Drosophila melanogaster (Springer-Verlag, Berlin; New York); Doe, 1992, Development 116:855-863). The pattern of production of these cells is largely determined by the activity of the proneural and neurogenic genes. Proneural genes predispose clusters of cells to a neural fate (reviewed in Skeath & Carroll, 1994, Faseb J. 8:714-21), but only a subset of cells in a cluster become neural precursors. This restriction is due to the action of the neurogenic genes, which mediate lateral inhibition--a type of inhibitory cell signaling by which a cell committed to a neural fate forces its neighbors either to remain uncommitted or to enter a non-neural pathway (Artavanis-Tsakonas & Simpson, 1991, Trends Genet. 7:403-408; Doe & Goodman, 1985, Dev. Biol. 111:206-219). Mutations leading to a failure of lateral inhibition cause an overproduction of neurons--the "neurogenic" phenotype (Lehmann et al., 1981, Roux's Arch. Dev. Biol. 190:226-229; Lehmann et al., Roux's Arch. Dev. Biol. 192:62-74). In Drosophila, the inhibitory signal is delivered by a transmembrane protein encoded by the Delta neurogenic gene, which is displayed by the nascent neural cells (Heitzler & Simpson, 1991, Cell 64:1083-1092). Neighboring cells express a transmembrane receptor protein, encoded by the neurogenic gene Notch (Fortini & Artavanis-Tsakonas, 1993, Cell 75:1245-1247). Delta has been identified as a genetic unit capable of interacting with the Notch locus (Xu et al., 1990, Genes Dev. 4:464-475).
Mutational analyses also reveal that the action of the neurogenic genes is pleiotropic and is not limited solely to embryogenesis. For example, ommatidial, bristle and wing formation, which are known also to depend upon cell interactions, are affected by neurogenic mutations (Morgan et al., 1925, Bibliogr. Genet. 2:1-226; Welshons, 1956, Dros. Inf. Serv. 30:157-158; Preiss et al., 1988, EMBO J. 7:3917-3927; Shellenbarger and Mohler, 1978, Dev. Biol. 62:432-446; Technau and Campos-Ortega, 1986, Wilhelm Roux's Dev. Biol. 195:445-454; Tomlison and Ready, 1987, Dev. Biol. 120:366-376; Cagan and Ready, 1989, Genes Dev. 3:1099-1112). Neurogenic genes are also required for normal development of the muscles, gut, excretory and reproductive systems of the fly (Muskavitch, 1994, Dev. Biol. 166:415-430).
Both Notch and Delta are transmembrane proteins that span the membrane a single time (Wharton et al., 1985, Cell 43:567-581; Kidd and Young, 1986, Mol. Cell. Biol. 6:3094-3108; Vassin, et al., 1987, EMBO J. 6:3431-3440; Kopczynski, et al., 1988, Genes Dev. 2:1723-1735) and include multiple tandem EGF-like repeats in their extracellular domains (Muskavitch, 1994, Dev. Biol. 166:415-430). The Notch gene encodes a .about.300 kd protein (we use "Notch" to denote this protein) with a large N-terminal extracellular domain that includes 36 epidermal growth factor (EGF)-like tandem repeats followed by three other cysteine-rich repeats, designated Notch/lin-12 repeats (Wharton, et al., 1985, Cell 43:567-581; Kidd and Young, 1986, Mol. Cell. Biol. 6:3094-3108; Yochem, et al., 1988, Nature 335:547-550). Molecular studies have lead to the suggestion that Notch and Delta constitute biochemically interacting elements of a cell communication mechanism involved in early developmental decisions (Fehon et al., 1990, Cell 61:523-534). Homologs are found in Caenorhabditis elegans, where the Notch-related gene lin-12 and the Delta-related gene lag-2 are also responsible for lateral inhibition (Sternberg, 1993, Current Biol. 3:763-765; Henderson et al., 1994, Development 120:2913-2924; Greenwald, 1994, Curr. Opin. Genet. Dev. 4:556-562). In vertebrates, several Notch homologs have also been identified (Kopan & Weintraub, 1993, J. Cell Biol. 121:631-641; Lardelli et al., 1994, Mech. Dev. 46:123-136; Lardelli & Lendahl, 1993, Exp. Cell Res. 204:364-372; Weinmaster et al., 1991, Development 113:199-205; Weinmaster et al., 1992, Development 116:931-941; Coffman et al., 1990, Science 249:1438-1441; Bierkamp & Campos-Ortega, 1993, Mech. Dev. 43:87-100), and they are expressed in many tissues and at many stages of development. Loss of Notch-1 leads to somite defects and embryonic death in mice (Swiatek et al., 1994, Genes Dev. 8:707-719; Conlon et al., Rossant, J. Development (J. Dev. 121:1533-1545), while constitutively active mutant forms of Notch-1 appear to inhibit cell differentiation in Xenopus and in cultured mammalian cells (Coffman et al., 1993, Cell 73:659-671; Kopan et al., 1994, Development 120:2385-2396; Nye et al., 1994, Development 120:2421-2430).
The EGF-like motif has been found in a variety of proteins, including those involved in the blood clotting cascade (Furie and Furie, 1988, Cell 53: 505-518). In particular, this motif has been found in extracellular proteins such as the blood clotting factors IX and X (Rees et al., 1988, EMBO J. 7:2053-2061; Furie and Furie, 1988, Cell 53: 505-518), in other Drosophila genes (Knust et al., 1987 EMBO J. 761-766; Rothberg et al., 1988, Cell 55:1047-1059), and in some cell-surface receptor proteins, such as thrombomodulin (Suzuki et al., 1987, EMBO J. 6:1891-1897) and LDL receptor (Sudhof et al., 1985, Science 228:815-822). A protein binding site has been mapped to the EGF repeat domain in thrombomodulin and urokinase (Kurosawa et al., 1988, J. Biol. Chem 263:5993-5996; Appella et al., 1987, J. Biol. Chem. 262:4437-4440).
Citation of references hereinabove shall not be construed as an admission that such references are prior art to the present invention.